Current work on the preparation of specific antisera for A-MuLV encoded proteins (1) harvested from animals rejecting syngeneic A-MuLV transformed cell lines will be continued. Such reagents have been used to characterize the major A-MuLV protein (called P120) as a cell surface antigen (1) and identify a normal cellular protein (called NCP150) in the lymphoid tissues of uninfected mice (2) that may represent the gene from which A-MuLV was derived. Further biochemical and cell biological comparison of the A-MuLV P120 protein to NCP150, and additional normal cellular proteins we may identify, will be done. Mutants and strain variants of A-MuLV will be isolated and characterized to attempt to define the transforming gene for this virus. Characterization of two recently isolated strain variants of A-MuLV suggest that large regions of the A-MuLV genome may be non-essential and can be deleted without loss of transforming activity. Further variants of this type could help to define the transforming region. A major effort will be made to isolate temperature-sensitive transformation mutants of A-MuLV to help define the transforming gene and for use in probing the nature of the A-MuLV lymphoid target cell. Agar transformation screening (3) used for isolating ts avian erythroblastosis virus mutants will be adopted for A-MuLV. Mutants which have altered A-MuLV cell surface antigen expression will be selected using anti-Abelson antisera as a specific probe. Finally, the very recent observation that A-MuLV P120 protein co-purifies with a protein kinase activity that specifically labels P120 but not unrelated proteins will be studied further. The identity of the kinase as an A-MuLV encoded protein (perhaps part of P120 itself) or as a host cell protein will be analyzed. The potential role for this kinase activity to modulate the transforming gene or serve as a transforming function itself is of great interest. Several other transforming viral systems have specific protein kinase activities associated with known transforming genes (4-7).